This invention relates to error detecting arrangements for digital transmission systems. It is particularly applicable to systems in which the line signals are already arranged in or can be converted into a format resulting in what may be termed constant accumulated disparity signals.
The term "constant accumulated disparity" is used to mean that over a given period of time (of sufficient duration) the ratio of digits of different values is such that the signals have a substantially constant d.c. content, which may be zero in some cases. One way of achieving zero accumulated disparity signals is to have normal binary coded p.c.m. signals random scrambled at a transmitter and descrambled at the receiver. Truly randomized signals have no d.c. content. Pseudo-randomized signals can be made to have, for all practical purposes, a negligible d.c. content. The same is true of scrambled binary coded signals which are subsequently translated into ternary coded signals.
According to the invention there is provided an error detecting arrangement for a digital transmission line system over which constant accumulated disparity signals are transmitted including at a transmitter means for modifying the line signals at intervals to introduce into the line signals control signals the digital values of which reflect the parity of predetermined digital signal events in the immediately preceding intervals, the control signals themselves having values which maintain the overall constant accumulated disparity of the transmitted signals, means at a subsequent point in the system for counting the predetermined digital signal events in the transmitted signals and means for determining a change in the d.c. content of the counting means output.
In one embodiment of the invention the modifying means comprises means for inserting additional parity check digits into the line signals.
In another embodiment of the invention where the line signals are in the form of digit code groups the modifying means comprises means for modifying or replacing selected code groups to provide different code groups, to indicate predetermined parity conditions.
Embodiments of the invention will now be described with reference to the accompanying drawings, in which: